Browsing by Author "K. Ram"

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A global 3-D CTM evaluation of black carbon in the Tibetan Plateau
(Copernicus GmbH, 2014) C. He; Q.B. Li; K.N. Liou; J. Zhang; L. Qi; Y. Mao; M. Gao; Z. Lu; D.G. Streets; Q. Zhang; M.M. Sarin; K. Ram
We systematically evaluate the black carbon (BC) simulations for 2006 over the Tibetan Plateau by a global 3-D chemical transport model (CTM) (GEOS-Chem) driven by GEOS-5 assimilated meteorological fields, using in situ measurements of BC in surface air, BC in snow, and BC absorption aerosol optical depth (AAOD). Using improved anthropogenic BC emission inventories for Asia that account for rapid technology renewal and energy consumption growth (Zhang et al., 2009; Lu et al., 2011) and improved global biomass burning emission inventories that account for small fires (van der Werf et al., 2010; Randerson et al., 2012), we find that model results of both BC in surface air and in snow are statistically in good agreement with observations (biases < 15%) away from urban centers. Model results capture the seasonal variations of the surface BC concentrations at rural sites in the Indo-Gangetic Plain, but the observed elevated values in winter are absent. Modeled surface-BC concentrations are within a factor of 2 of the observations at remote sites. Part of the discrepancy is explained by the deficiencies of the meteorological fields over the complex Tibetan terrain. We find that BC concentrations in snow computed from modeled BC deposition and GEOS-5 precipitation are spatiotemporally consistent with observations (rCombining double low line 0.85). The computed BC concentrations in snow are a factor of 2-4 higher than the observations at several Himalayan sites because of excessive BC deposition. The BC concentrations in snow are biased low by a factor of 2 in the central plateau, which we attribute to the absence of snow aging in the CTM and strong local emissions unaccounted for in the emission inventories. Modeled BC AAOD is more than a factor of 2 lower than observations at most sites, particularly to the northwest of the plateau and along the southern slopes of the Himalayas in winter and spring, which is attributable in large part to underestimated emissions and the assumption of external mixing of BC aerosols in the model. We find that assuming a 50% increase of BC absorption associated with internal mixing reduces the bias in modeled BC AAOD by 57% in the Indo-Gangetic Plain and the northeastern plateau and to the northeast of the plateau, and by 16% along the southern slopes of the Himalayas and to the northwest of the plateau. Both surface BC concentration and AAOD are strongly sensitive to anthropogenic emissions (from China and India), while BC concentration in snow is especially responsive to the treatment of BC aerosol aging. We find that a finer model resolution (0.5° × 0.667° nested over Asia) reduces the bias in modeled surface-BC concentration from 15 to 2%. The large range and non-homogeneity of discrepancies between model results and observations of BC across the Tibetan Plateau undoubtedly undermine current assessments of the climatic and hydrological impact of BC in the region and thus warrant imperative needs for more extensive measurements of BC, including its concentration in surface air and snow, AAOD, vertical profile and deposition. © Author(s) 2014. CC Attribution 3.0 License.
Aerosol optical properties and radiative effects over Manora Peak in the Himalayan foothills: Seasonal variability and role of transported aerosols
(Elsevier, 2015) A.K. Srivastava; K. Ram; Sachchidanand Singh; Sanjeev Kumar; S. Tiwari
The higher altitude regions of Himalayas and Tibetan Plateau are influenced by the dust and black carbon (BC) aerosols from the emissions and long-range transport from the adjoining areas. In this study, we present impacts of advection of polluted air masses of natural and anthropogenic emissions, on aerosol optical and radiative properties at Manora Peak (~2000m amsl) in central Himalaya over a period of more than two years (February 2006-May 2008). We used the most updated and comprehensive data of chemical and optical properties available in one of the most climatically sensitive region, the Himalaya, to estimate atmospheric radiative forcing and heating rate. Aerosol optical depth (AOD) was found to vary from 0.04 to 0.45 with significantly higher values in summer mainly due to an increase in mineral dust and biomass burning aerosols due to transport. In contrast, single scattering albedo (SSA) varied from 0.74 to 0.88 with relatively lower values during summer, suggesting an increase in absorbing BC and mineral dust aerosols. As a result, a large positive atmospheric radiative forcing (about 28±5 Wm-2) and high values of corresponding heating rate (0.80±0.14 Kday-1) has been found during summer. During the entire observation period, radiative forcing at the top of the atmosphere varied from -2 to +14 Wm-2 and from -3 to -50 Wm-2 at the surface whereas atmospheric forcing was in the range of 3 to 65 Wm-2 resulting in a heating rate of 0.1-1.8 Kday-1. © 2014 Elsevier B.V.
Characterization of carbonaceous aerosols over Delhi in Ganga basin: Seasonal variability and possible sources
(Springer Verlag, 2014) A.K. Srivastava; D.S. Bisht; K. Ram; S. Tiwari; Manoj K. Srivastava
The mass concentration of carbonaceous species, organic carbon (OC), and elemental carbon (EC) using a semicontinuous thermo-optical EC-OC analyzer, and black carbon (BC) using an Aethalometer were measured simultaneously at an urban mega city Delhi in Ganga basin from January 2011 to May 2012. The concentrations of OC, EC, and BC exhibit seasonal variability, and their concentrations were ~2 times higher during winter (OC 38.1 ± 17.9 μg m-3, EC 15.8 ± 7.3 μg m-3, and BC 10.1 ± 5.3 μg m-3) compared to those in summer (OC 14.1 ± 4.3 μg m-3, EC 7.5 ± 1.5 μg m-3, and BC 4.9 ± 1.5 μg m-3). A significant correlation between OC and EC (R = 0.95, n = 232) indicate their common emission sources with relatively lower OC/EC ratio (range 1.0-3.6, mean 2.2 ± 0.5) suggests fossil fuel emission as a major source of carbonaceous aerosols over the station. On average, mass concentration of EC was found to be ~38 % higher than BC during the study period. The measured absorption coefficient (babs) was significantly correlated with EC, suggesting EC as a major absorbing species in ambient aerosols at Delhi. Furthermore, the estimated mass absorption efficiency (σabs) values are similar during winter (5.0 ± 1.5 m2 g-1) and summer (4.8 ± 2.8 m2 g-1). Significantly high aerosol loading of carbonaceous species emphasize an urgent need to focus on air quality management and proper impact assessment on health perspective in these regions. © 2014 Springer-Verlag Berlin Heidelberg.
Chemical characterization of rainwater at a high-altitude site “Nainital” in the central Himalayas, India
(Springer Verlag, 2017) Deewan Singh Bisht; A.K. Srivastava; H. Joshi; K. Ram; N. Singh; M. Naja; M.K. Srivastava; S. Tiwari
The present study investigates the chemical composition of rainwater (RW) from a high-altitude site “Nainital” (1958 m above msl) in the central Himalaya region, to understand the influence of local, regional, and long-range transport of pollutants. A total of 55 (2 in pre-monsoon and 53 in monsoon) RW samples were collected during the study period (June–September 2012) and were analyzed for major anions and cations using an ion chromatograph. The pH of precipitation events ranged from 4.95 to 6.50 (average 5.6 ± 0.3) was observed during the monsoon period (near to the acidic), whereas during the pre-monsoon, the pH was 6.25 ± 0.49 (alkaline) over the study region; it is due the mixture of anthropogenic as well as the natural chemical constituents. The average ionic concentration (sum of measured chemical constituents) was ∼3 times higher during the pre-monsoon (986 ± 101 μeq/1) compared to that in the monsoon season (373 ± 37 μeq/1). This is mainly due to the presence of more natural aerosols in the pre-monsoon season which is also reflected in the pH of rainwater (average 6.25 ± 0.50) as well as ionic concentration. The chemical composition suggests that Ca2+ was the major contributor (34%) among cations, followed by Na+ (10%), K+ (8%), and Mg2+ (9%), whereas Cl−, NO3 −, and SO4 2− contributed ∼13, 11, and 9%, respectively, among anions. The average ratio of acidic species (SO4 2−/NO3 −) is 1.56, suggesting 61 and 39% contribution of SO4 2− and NO3 −, respectively, which is very close to the estimated contribution of H2SO4 (60–70%) and HNO3 (30–40%) in the precipitation samples. Neutralization factors for Ca2+, Mg2+, and NH4 + in RW at Nainital are 4.94, 1.21, and 0.37, respectively, indicating their crucial role in neutralization of acidic species. The non-sea-salt (NSS) contribution to total Ca2+, K+, and Mg2+ is estimated to be ∼98, 97, and 74%, respectively, suggesting the dominance of crustal sources for cations. In contrast, the NSS contribution to the total Cl− and SO4 2− is 16 and 69% indicating their anthropogenic origin, respectively. Principle component analysis also suggests that the first factor (i.e., natural sources, mainly dust, and sea-salts) accounts for ∼33% variance, whereas the second factor (i.e., fossil fuel and biomass burning) accounts for ∼18% variance of the measured ionic composition. The remaining contributions are attributed to the mixed emission sources and transport of pollutants from Indo-Gangetic Plain (IGP) and western parts of India. The results of the present study reveal a significant contribution of crustal and anthropogenic sources in the RW and neutralization processes in the central Himalaya. © 2016, Springer-Verlag Berlin Heidelberg.
Dual carbon isotope characterization of total organic carbon in wintertime carbonaceous aerosols from northern India
(Blackwell Publishing Ltd, 2016) Srinivas Bikkina; August Andersson; M.M. Sarin; R.J. Sheesley; E. Kirillova; R. Rengarajan; A.K. Sudheer; K. Ram; Örjan Gustafsson
Large-scale emissions of carbonaceous aerosols (CA) from South Asia impact both regional climate and air quality, yet their sources are not well constrained. Here we use source-diagnostic stable and radiocarbon isotopes (δ13C and Δ14C) to characterize CA sources at a semiurban site (Hisar: 29.2°N, 75.2°E) in the NW Indo-Gangetic Plain (IGP) and a remote high-altitude location in the Himalayan foothills (Manora Peak: 29.4°N, 79.5°E, 1950 m above sea level) in northern India during winter. The Δ14C of total aerosol organic carbon (TOC) varied from -178‰ to -63‰ at Hisar and from -198‰ to -1‰ at Manora Peak. The absence of significant differences in the 14C-based fraction biomass of TOC between Hisar (0.81 ± 0.03) and Manora Peak (0.82 ± 0.07) reveals that biomass burning/biogenic emissions (BBEs) are the dominant sources of CA at both sites. Combining this information with δ13C, other chemical tracers (K+/OC and SO42-/EC) and air mass back trajectory analyses indicate similar source regions in the IGP (e.g., Punjab and Haryana). These results highlight that CA from BBEs in the IGP are not only confined to the atmospheric boundary layer but also extend to higher elevations of the troposphere, where the synoptic-scale circulations could substantially influence their abundances both to the Himalayas and over the downwind oceanic regions such as the Indian Ocean. Given the vast emissions of CA from postharvest crop residue combustion practices in the IGP during early Northeast Monsoon, this information is important for both improved process and model understanding of climate and health effects, as well as in guiding policy decision aiming at reducing emissions. © 2016. American Geophysical Union. All Rights Reserved.
Relationship of arsenic accumulation with irrigation practices and crop type in agriculture soils of Bengal Delta, India
(Springer Verlag, 2019) S.H. Farooq; D. Chandrasekharam; W. Dhanachandra; K. Ram
The present study investigates the current state and distribution of As in the upper soil horizons (i.e. rhizospheric zone, 0–15 cm) of two different agricultural fields: (1) paddy-cultivating agricultural field that was irrigated with groundwater containing 137 µg/L of As and (2) wheat-cultivating agricultural field that was irrigated with groundwater having 67.3 µg/L of arsenic. Results clearly indicate different levels of As accumulation in the upper soil horizons of both profiles. In paddy field, although significantly higher quantity of As-contaminated groundwater was used for irrigation, still lesser than expected As concentration in soils was found [average As concentration 16.0 mg/kg (measured) vs. 29.0 mg/kg (calculated)]. The imbalance between higher influx of As and its relatively lower accumulation in soils indicates the existence of a mechanism (organic carbon mechanism, elaborated in the main text) that is responsible for continuous removal of As, and ultimately prevents the expected shoot-up of As in the paddy soils. On the other hand, although lesser quantity of less contaminated groundwater is used in wheat field, still wheat field soils show relatively higher As accumulation [average As concentration 22.5 mg/kg (measured) vs. 12.2 mg/kg (calculated)]. Such accumulation of As happens when there is continuous influx of As through irrigation water and/or other sources, and an effective (natural) mechanism to remove As from the wheat soil is absent. Adoption of distinct harvesting methods is responsible for existence of different mechanisms in paddy and wheat fields, which ultimately cause the differential accumulation of As in paddy and wheat soils. © 2019, The Author(s).
Temporal variability in aerosol characteristics and its radiative properties over Patiala, northwestern part of India: Impact of agricultural biomass burning emissions
(Elsevier Ltd, 2017) D. Sharma; A.K. Srivastava; K. Ram; A. Singh; D. Singh
A comprehensive measurements of aerosol optical depth (AOD), particulate matter (PM) and black carbon (BC) mass concentrations have been carried out over Patiala, a semi-urban site in northwest India during October 2008 to September 2010. The measured aerosol data was incorporated in an aerosol optical model to estimate various aerosol optical parameters, which were subsequently used for radiative forcing estimation. The measured AOD at 500 nm (AOD500) shows a significant seasonal variability, with maximum value of 0.81 during post-monsoon (PoM) and minimum of 0.56 during winter season. The Ångström exponent (α) has higher values (i.e. more fine-mode fraction) during the PoM/winter periods, and lower (i.e. more coarse-mode fraction) during pre-monsoon (PrM). In contrast, turbidity coefficient (β) exhibits an opposite trend to α during the study period. BC mass concentration varies from 2.8 to 13.9 μg m−3 (mean: 6.5 ± 3.2 μg m−3) during the entire study period, with higher concentrations during PoM/winter and lower during PrM/monsoon seasons. The average single scattering albedo (SSA at 500 nm) values are 0.70, 0.72, 0.82 and 0.75 during PoM, winter, PrM and monsoon seasons, respectively. However, inter-seasonal and inter-annual variability in measured aerosol parameters are statistically insignificant at Patiala. These results suggest strong changes in emission sources, aerosol composition, meteorological parameters as well as transport of aerosols over the station. Higher values of AOD, α and BC, along with lower SSA during PoM season are attributed to agriculture biomass burning emissions over and around the station. The estimated aerosol radiative forcing within the atmosphere is positive (i.e. warming) during all the seasons with higher values (∼60 Wm−2) during PoM–08/PoM–09 and lower (∼40 Wm−2) during winter–09/PrM–10. The present study highlights the role of BC aerosols from agricultural biomass burning emissions during post-monsoon season for atmospheric warming at Patiala. © 2017 Elsevier Ltd
What caused severe air pollution episode of November 2016 in New Delhi?
(Elsevier Ltd, 2020) V.P. Kanawade; A.K. Srivastava; K. Ram; E. Asmi; V. Vakkari; V.K. Soni; V. Varaprasad; C. Sarangi
In recent years, South Asia is experiencing severely degraded air quality, with particulate matter less than 2.5 μm (PM2.5) reaching unprecedented high levels. Here, we investigate a severe air pollution episode (SAPE) witnessed in New Delhi during 1–7 November 2016. This was a very unusual air pollution episode wherein air quality index exceeded >500 and was persistent for about a week encapsulating the entire Indo-Gangetic Plain (IGP). We demonstrate that a stagnant weather condition was the dominant cause of the SAPE. Mean concentration of PM2.5 in New Delhi before, during, and after the SAPE were 142 μg/m3, 563 μg/m3, and 240 μg/m3, respectively. Satellite-based aerosol optical depth (AOD), ultraviolet-aerosol index (UV-AI) and surface carbon monoxide (CO) concentrations also showed significant enhancements over large locale spatially by about 50–70% during the SAPE. A large and simultaneous increase in UV-AI and CO downwind of a large number of fire hotspots (Punjab and Haryana) is a clear indication of biomass burning aerosols. Analysis of absorption Ångström exponent further substantiates this finding, showing a large fraction of light absorbing carbonaceous-type aerosols. Radiosonde observations clearly showed that stagnant atmospheric conditions led to SAPE in New Delhi by allowing pollution to accumulate and persist in the near-surface environment. As a result new particle formation was suppressed due to very high pre-existing aerosol concentrations during the SAPE. The heating rate induced by light absorbing aerosols into an atmospheric layer during SAPE was also very high (3.1 ± 0.7 K/day). These findings will help in understanding air quality and climate effects, as well as in formulating policies to mitigate these complex pollution episodes in an anthropogenic future. © 2019 Elsevier Ltd